I used to grow from sets and found similar problems to you. Now I grow from seed and nearly all of those problems disappeared, with the exception that about 10% don't grow very big .... which is not really a problem as I pickle those ones.
I start them off multiple sown in modules usually 3 seeds to a module and that stops them growing too big.

When the growth falls over, as they did last week, I pull them up and leave them on the ground to ripen a bit and then put them in the greenhouse to ripen fully

I am going to have to give onion seeds another go, I let the slugs eat the ones I sowed last year and reverted to sets. The problem I have had this year is that about 50% of them have bolted, I usually have a few that bolt, but this year they are just doing really badly, I think the erratic weather conditions are to blame though, three nights this week have been 4 celsius, which is ridiculous.

I transferred my onions to the greenhouse today to dry off for a while.
These were multiple sown in 24 cells at the beginning of January. There are a few pickling sized ones, but most are between 2" & 3" in diameter, just the right size for us.

We usually grow sets but OH decided to sow seeds this year and we planted them out at the same time as the leeks - and they look remarkably like them too, long straight stems (nibbled a bit on top) but no bulbs - will we be able to eat them? Or do we need to over winter them somehow? I'm sure the packet said to sow them in spring. They're Bedfordshire, by the way.

We're not ready to harvest them yet but I can't see them filling out between now and September - ish, when we usually dig them up.

Maggie

Never doubt that you can change history. You already have. Marge Piercy

They all look like leeks until midsummer, then they start to swell (long day onions).
I believe long day onions need about 15 hours of daylight and short day need about 10 hours, so maybe your onions are waiting for the day length to start shortening to less that 15 hours.
They are also greedy and like a dose of liquid fertiliser now and again, or at least mine do (did)

dave45 wrote:I plant onions from sets, both red and yellow, in the same bit of ground, and subject to the same weather...

So why do some onions:
a) grow and bulb just fineYou got all the conditions right, the weather right and grew from good stock
b) hardly grow at allCould be lack of fertility, could be dodgy sets or seeds if you go for cheap shop ones - been there, done that and don't do it now
c) grow vigorously with fat main stems but fail to bulb much at allThis is usually caused by too much nitrogen & not enough potash, or over feeding late in the season. Some believe that planting onion seeds too deeply will also cause 'bullnecks'.
d) go to a full bolt and attempt to flower ???That's warm weather at the wrong time in the growth cycle

And is an onion "ready" when the the green bits fall over at right angles?Yes but I wait a few days till the green bits start to change colour so that I know the bulb is ripe

daffodiltulip wrote:I bought a pack of Electric onion sets today. A few have sprouted (1cm to a few cm). Should I plant these so that the green bits are above ground completely or partially covered by soil?